The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Paper mill slurry stock supplied to the forming fabric of a paper machine is made up of fibers and solids in an aqueous solution containing generally from about 99 to about 99.9 percent water. The aim of a paper maker is to mix the slurry stock thoroughly in the head box of a paper making machine so that the fibers will be uniformly dispersed. Despite this attempt, the fibers often tend to agglomerate in the head box and emerge from the slice in clumps or floes and the slurry stock is deposited on the forming fabric in this condition. If these flocs or fibers remain undispersed, the finished paper will not be of uniform density.
The forming fabric, as used on typical paper making machines, is an open mesh belt of woven cloth. The warp and weft strands of the cloth may be a metal, for example bronze or stainless steel or a plastic material, for instance polyester in multifilament or monofilament form.
Several devices have been used to redistribute fibers in the slurry stock after it has been transferred to the forming fabric during a dewatering process. U.S. Pat. No. 3,874,998 to Johnson discloses a series of replaceable blade elements or drainage foils disposed under the forming fabric to reduce flocculation. The foils disclosed by Johnson include machined grooves or channels in a surface of the foil to provide pressure pulses through the forming fabric which produces controlled agitation of the slurry stock. One drawback of the foil disclosed by Johnson is the channels formed in the foil blades have fixed dimensions, thus, even if a particular foil blade works well with one grade of paper and processing speed, the same blade might not have an appropriate channel for operation with another grade or paper or processing speed.
U.S. Pat. No. 4,838,996 to Kallmes discloses a hydrofoil blade for use in a paper making machine wherein a plurality of variously angulated surfaces is provided for producing turbulence having controllable scale and intensity while independently controlling the rate of dewatering. The Kallmes foil includes a trailing edge of the foil designed to fall away from the forming fabric, thus the foil does not force the stock back through the forming fabric. Similar to the Johnson device, the Kallmes design has a fixed profile that may work well with one grade of paper and speed but not across all grades of paper and machines.
U.S. Pat. No. 5,169,500 to Mejdell teaches an adjustable angle foil for a paper making machine in which a rigid foil member is pivoted by a cam actuated adjustment mechanism to change the foil angle. Similar to the Kallmes foil, adjustment of the foil disclosed by Mejdell may cause a trailing edge to move away from a forming fabric which may reduce a volume of the stock being forced back through the forming fabric.
Each of the above-mentioned devices are used to reduce floccing in a paper making process however, none of the prior art devices are sufficiently adjustable to suit the changing variety of paper grades, weights and processing speeds currently delivered by a typical paper making machine. Accordingly, using the above-described foil blades, a paper maker is often tasked with continuously removing and replacing foil blades of varied specifications in an attempt to maintain high quality paper of various grades and made with differing processing speeds.
It is an object of the present teachings to provide an adjustable pulse generating foil apparatus for a papermaking machine that overcomes the shortcomings of prior art foil devices.